A quantitative structural analysis of the low temperature phase of lithium-7

Calculations in 7Li have been carried out for the differential coherent elastic scattering cross-section of thermal neutrons at a fixed scattering angle θo(= 148°) and varying the energy of incident neutrons (or corresponding d-spacings). The results are reported, as variations of the ratio Σocoh(θo, d)/Σocoh(θo) with d-spacing, at atmospheric pressure and at two temperatures, 80 and 20 K. The calculations at 80 K correspond to BCC lithium whereas at 20 K all the possible crystallographic structures of polytype lithium (BCC, 9R, HCP and FCC) were considered. On comparing our results with the time-of-flight (TOF) measurements of Berliner et al. (Physical Review, B40, 12086, 1989) we find that up to 80 K lithium-7 does not undergo any martensitic transformation whereas the structural state at 20 K can be described by the co-existence of the original BCC phase with 9R, HCP and FCC closed packed structures. A quantitative structural analysis of the various peaks in the observed diffraction patterns of Berliner et al. is presented.